1. Field of the Invention
The present invention relates to an electric motor and a method for determining a position or rotation speed of a motor and more particularly to a motor and method in which brushes and a commutator of a motor detect a rotary motion and a position of a rotor.
2. Description of the Related Art
For many applications involving small electric motors, it is advantageous if, in addition to producing a rotary motion, an electric motor also can supply information about its current state or position. In order to accomplish this, it is known, for example, to use the excitation circuit of the electric motor as a “ripple counter” or as an asynchronous counter.
Such an electric motor is disclosed in, for example, EP 1 316 136. In this electric motor, two brushes are arranged to alternately contact six lamellae or segments of the commutator. In this case, the brushes in the commutator are arranged opposite to each other, i.e. are offset by 180°, so that the brushes are in contact with respective opposite segments of the commutator. During the rotary motion of the commutator, a periodic change takes place from one segment to the adjacent segment. During this time, each brush is in electrical contact with two segments simultaneously. The corresponding switching or commutation therefore occurs simultaneously in both brushes. Accordingly, there is always a total of either one segment or two segments in contact with each of the brushes. By detecting the current of the motor, the number of segments in contact with brushes can be speculated, and the rotating speed and position of the rotor is possible to be calculated. However, in order to increase the torque, a motor with four magnets is always used. And in this situation, the vibration and noises of the motor is big if the number of coils is an integral multiple of the number of brushes, and the numbers of segments in connection with both brushes change at the same time. The present invention therefore is based on the task of reducing the vibration and noises, with the generated current signal easy to be detected to determine the rotating speed and position of the motor.